1. Field of the Invention
The present invention relates to an electron gun used in a lithography process for manufacturing a semiconductor device, an electron beam exposure apparatus provided with the electron gun, and to an exposure method.
2. Description of the Prior Art
In order to enhance throughput, a recent electron beam exposure apparatus is provided with a mask having a variable rectangular aperture formed therein, or with multiple mask patterns. One of the mask patterns is firstly selected by means of beam deflection, and the selected pattern is then transferred onto a wafer through exposure. An exposure method employing the above-mentioned multiple mask patterns is achieved by an electron beam exposure device that is configured to perform block exposure. In the block exposure, patterns are transferred onto a sample surface by the following procedures. Specifically, a beam is irradiated on a pattern region selected from multiple patterns disposed on a mask by means of beam deflection so that a cross section of the beam is formed into a shape of the pattern. Furthermore, the beam that has passed through the mask is deflected back by a deflector located on a later stage and is transferred onto the sample surface after the scale of the pattern is reduced to a certain reduction ratio which is determined by an electron optical system.
Incidentally, ensuring linewidth accuracy is also an important factor for the above-mentioned type of exposure apparatus in order to enhance throughput. To ensure linewidth accuracy, intensity of an electron beam emitted from an electron gun must not be changed over time. In a case where the intensity of the electron beam is weakened due to the change over time, the degree of exposure is gradually reduced. An increase in exposure time to supplement the weakened electron beam due to the change over time may lead not only to complication of control but also to deterioration in the throughput.
In general, methods of emitting electrons from an electron gun are classified broadly into a thermal electron emission type method and a field emission type method. The thermal electron emission type electron gun includes a cathode configured to emit electrons by heating, a Wehnelt cylinder configured to generate an electron beam flux with converging the electrons emitted from the cathode, and an anode configured to accelerate the converged electron beam.
With the use of the above-described thermal electron emission type electron gun, a material constituting an electron source (a chip) used in the electron gun loses its quantity by sublimation or evaporation along with the emission of the electrons. Accordingly, a phenomenon of deformation of an electron emitter occurs. Various countermeasures have been studied to prevent the above-mentioned phenomenon. For example, Japanese Patent Application Publication No. Hei 8-184699 discloses an electron gun in which a chip is covered by a two-layer structure consisting of tungsten (W) and rhenium (Re) to suppress depletion of a chip.
As described above, with the use of the thermal electron emission type electron gun, not only the chip that constitutes the electron gun may emit the electrons but also the materials themselves constituting a chip may sublimate in some cases. The above mentioned sublimation of a chip is considered to occur because the temperature is heated up to the extent equal to or higher than the sublimation start temperature of the electron generating material.
Due to this sublimation, the shape of the chip for emitting the electrons is changed. Accordingly, such deformation prevents uniform irradiation of a variable rectangular beam or a block pattern beam, which causes the intensity reduction of the emitted electron beam. For example, with the use of a thermal electron emission type electron gun constituting a chip made of lanthanum hexaboride (LaB6), at a temperature equal to 1500° C., the chip sublimates in an amount of 10 μm in one month.
Moreover, the chip material such as LaB6 or cerium hexaboride (CeB6) is attached to a rear surface of a grid as a result of the above-mentioned sublimation. The attached material may grow to a whisker. Then with the charged electrons thereon, a micro discharge may occur. Occurrence of this micro discharge may cause instabilities of the electron beam quantity and of a radiating position, thereby making it impossible to properly use an electron beam exposure apparatus. Moreover, the throughput may be deteriorated due to the time consumption for adjustment or other operations. The most serious problem is that the pattern drawn during the occurrence of the micro discharge may lead a loss of reliability. Therefore, the eradication of the micro discharge around the electron gun is indispensable to establish a high reliability of the electron beam exposure apparatus. In other words, the reduction in quantity of the material sublimation for the electron gun is the indispensable factor for the development to establish a high reliability as well as a high stability.
Although Japanese Patent Application Publication No. Hei 8-184699, discloses the technique to suppress depletion of the chip by covering the surface of the chip with the two-layer structure made of tungsten (W) and rhenium (Re), this technique is unable to prevent the change in the shape of a chip attributable to sublimation of an electron emission surface not covered with the two-layer structure.